This invention deals with substantially fluorinated but not perfluorinated ionomers, and related ionic and nonionic monomers, having pendant groups containing fluorosulfonyl methide or fluorosulfonyl imide derivatives and univalent metal salts thereof, and with the uses of said ionomers in electrochemical applications such as batteries, fuel cells, electrolysis cells, ion exchange membranes, sensors, electrochromic windows, electrochemical capacitors, and modified electrodes. Certain compositions of the invention are also useful as strong acid catalysts.
Copolymers of vinylidene fluoride (VF2) with vinyl alkoxy sulfonyl halides are known in the art.
The disclosures in Ezzell et al. (U.S. Pat. No. 4,940,525) encompass copolymers of VF2 with vinyl ethoxy sulfonyl fluorides containing one ether linkage. Disclosed is a process for emulsion polymerization of tetrafluoroethylene (TFE) with the vinyl ethoxy comonomer.
Connolly et al. (U.S. Pat. No. 3,282,875) disclose the terpolymer of VF2 with perfluorosulfonyl fluoride ethoxy propyl vinyl ether (PSEPVE) and hexafluoropropylene (HFP). They broadly teach an emulsion polymerization process said to be applicable to copolymerization of vinyl ethers with any ethylenically unsaturated comonomer, with greatest applicability to fluorinated monomers.
DesMarteau (U.S. Pat. No. 5,463,005), incorporated herein by reference, discloses substituted perfluoro-olefins of the formula 
where Xxe2x95x90C or N, Zxe2x95x90H, K, Na, or Group I or II metal, R=one or more fluorocarbon groups including fluorocarbon ethers and/or sulfonyl groups and/or perfluoro non-oxy acid groups, Y=perfluoroalkyl or F, and m=0 or 1. Further disclosed by DesMarteau are copolymers formed by aqueous emulsion polymerization of the sodium salt form of (I) with tetrafluoroethylene. Further disclosed are compositions consisting of the acid-form of the imide copolymer of DesMarteau in combination with dimethylformamide (hereinafter DMF) to provide a conductive composition. Membranes or films of the acid imide polymer are cast from solution. Copolymers of the substituted perfluoroolefins with VF2 are not disclosed in U.S. Pat. No. 5,463,005.
Armand (U.S. Pat. No. 4,818,644) discloses metal salts based on anions having the structure Rfxe2x80x94SO2CRxe2x80x94SO2Rxe2x80x2f where Rf and Rxe2x80x2f are perfluorinated groups having from 1 to 10 carbon atoms and R is a hydrogen or an alkyl group having from 1 to 30 carbon atoms. The lithium salts of these compounds are useful in combination with organic solvents or macromolecular solvents for making electrolyte solutions for lithium batteries. Armand et al. further disclose (EP 0 850 921) salts and ionomeric polymers derived from malononitrile Zxe2x80x94C(CN)2 where Z represents an electron-withdrawing group and Z can also contain a polymerizable function. Ionomers based on these compounds are disclosed having styrenic or vinyl functional groups for polymerization. Copolymers of these monomers with substantially fluorinated monomers such as VF2 are not disclosed.
Xue, Ph.D. thesis, Clemson University, 1996, discloses reactions of the type RSO2NHX with Rxe2x80x2SO2Y with Xxe2x95x90H, Na and Yxe2x95x90Cl, F to form RSO2N(M)SO2Rxe2x80x2, where R and Rxe2x80x2 are perfluorinated groups, in the presence of MF with Mxe2x95x90Cs, K or in the presence of Na2CO3 if Xxe2x95x90Na and Yxe2x95x90Cl to form monomers represented by the formula
CF2xe2x95x90CFxe2x80x94OCF2CF(CF3)OCF2CF2SO2NMSO2Rf
and copolymers thereof with tetrafluoroethylene.
Armand et al, EP0850921A1 and EP0850920A1, provide a tremendous list of imide- and methide-containing ionic species, including polymers incorporating them. However, no means for making these compositions is provided, and no distinction is made among the compounds from the standpoint of utility. No disclosure is made of the particular utility and surprising attributes of the compositions of the present invention.
The present invention provides for an ionic polymer (ionomer) comprising monomer units of VF2 and further comprising 0.5-50 mol-% of monomer units having pendant groups comprising the radical represented by the formula
xe2x80x94(OCF2CFR)aOCF2(CFRxe2x80x2)bSO2Xxe2x88x92(M+)(Y)(Z)cxe2x80x83xe2x80x83(I)
wherein
R and Rxe2x80x2 are independently selected from F, Cl or a perfluoroalkyl group having 1 to 10 carbon atoms optionally substituted by one or more ether oxygens;
a=0, 1 or 2;
b=0 to 6;
M+ is H+ or a univalent metal cation;
X is C or N with the proviso that c=1 when X is C and c=0 when X is N; when c=1, Y and Z are electron-withdrawing groups selected from the group consisting of CN, SO2Rf, SO2R3, P(O)(OR3)2, CO2R3, P(O)R32, C(O)RfC(O)R3, and cycloalkenyl groups formed therewith
wherein Rf is a perfluoroalkyl group of 1-10 carbons optionally substituted with one or more ether oxygens;
R3 is an alkyl group of 1-6 carbons optionally substituted with one or more ether oxygens, or an aryl group optionally further substituted;
Y and Z are the same or different;
or, when c=0, Y may be an electron-withdrawing group represented by the formula xe2x80x94SO2Rfxe2x80x2 where Rfxe2x80x2 is the radical represented by the formula xe2x80x94(Rfxe2x80x3SO2Nxe2x88x92(M+)SO2)mRfxe2x80x2xe2x80x3 where m=0 or 1, and Rfxe2x80x3 is xe2x80x94CnF2nxe2x80x94 and Rfxe2x80x2xe2x80x3 is xe2x80x94CnF2n+1 where n=1-10, optionally substituted with one or more ether oxygens.
The present invention further provides for an ethylenically unsaturated composition represented by the formula
CF2xe2x95x90CF(OCF2CFR)aOCF2(CFRxe2x80x2)bSO2Cxe2x88x92(M+)(Y)(Z)xe2x80x83xe2x80x83(II)
wherein
R and Rxe2x80x2 are independently selected from F, Cl or a perfluoroalkyl group having 1 to 10 carbon atoms optionally substituted with one or more ether oxygens;
a=0, 1 or 2;
b=0 to 6;
M+ is H+ or a univalent metal cation;
Y and Z are electron-withdrawing groups selected from the group consisting of CN, SO2R3, P(O)(OR3)2, CO2R3, P(O)R32, C(O)RfC(O)R3, and cycloalkenyl groups formed therewith wherein Rf is a perfluoroalkyl group of 1-10 carbons optionally substituted with one or more ether oxygens;
R3 is an alkyl group of 1-6 carbons optionally substituted with one or more ether oxygens, or an aryl group optionally further substituted;
Y and Z are the same or different.
The present invention further provides a method for making a methide ionomer the method comprising, combining in an inert organic liquid at a temperature in the range of 0-150xc2x0 C. a copolymer comprising monomer units of VF2 and 0.5-50 mol-% of monomer units represented by the formula:
CF2xe2x95x90CF(OCF2CFR)aOCF2(CFRxe2x80x2)bSO2Fxe2x80x83xe2x80x83(III)
wherein R and Rxe2x80x2 are independently selected from F, Cl or a perfluoroalkyl group having 1 to 10 carbon atoms optionally substituted with one or more ether oxygens, a=0, 1 or 2, and b=0 to 6; with a carbanion derived from a methylene compound represented by the formula CH2YZ wherein Y and Z are electron-withdrawing groups selected from the group consisting of CN, SO2Rf, SO2R3, P(O)(OR3)2, CO2R3, P(O)R32, C(O)RfC(O)R3, and cycloalkenyl groups formed therewith, wherein Rf is a perfluoroalkyl group of 1-10 carbons, optionally substituted with one or more ether oxygens, R3 is an alkyl group of 1-6 carbons, optionally substituted with one or more ether oxygens or an aryl group optionally further substituted; and wherein Y and Z may be either the same or different to form a reaction mixture; reacting said reaction mixture until the desired degree of conversion has been achieved; and, removing the majority of said organic liquid.
The present invention further provides a method for making a methide composition the method comprising, combining an inert organic solvent at a temperature in the range of 0-100xc2x0 C. a composition represented by the formula
CF2Axe2x80x94CFA(OCF2CFR)aOCF2(CFRxe2x80x2)bSO2Fxe2x80x83xe2x80x83(IV)
wherein A is Br or Cl, R and Rxe2x80x2 are independently selected from F, Cl or a perfluoroalkyl group having 1 to 10 carbon atoms, a=0, 1 or 2, and b=0 to 6; with a carbanion derived from a methylene compound represented by the formula CH2YZ wherein Y and Z are electron-withdrawing groups selected from the group consisting of CN, SO2Rf, SO2R3, P(O)(OR3)2, CO2R3, P(O)R32, C(O)RfC(O)R3, and cycloalkenyl groups formed therewith, wherein Rf is a perfluoroalkyl group of 1-10 carbons, optionally substituted with one or more ether oxygens, R3 is an alkyl group of 1-6 carbons, optionally substituted with one or more ether oxygens or an aryl group optionally further substituted; and wherein Y and Z may be either the same or different to form a reaction mixture; reacting said mixture until the desired degree of conversion has been achieved; and, removing majority of said organic liquid.
The present invention further provides a process for forming an ionomer, the process comprising combining in an aqueous reaction medium VF2 with an ionic monomer represented by the formula
CF2xe2x95x90CFxe2x80x94(OCF2CFR)aOCF2(CFRxe2x80x2)bSO2Xxe2x88x92(M+)(Y)(Z)cxe2x80x83xe2x80x83(II)
wherein
R and Rxe2x80x2 are independently selected from F, Cl or a perfluoroalkyl group having 1 to 10 carbon atoms optionally substituted with one or more ether oxygens;
a=0, 1 or 2;
b=0 to 6;
M+ is H+ or a univalent metal cation;
X is C or N with the proviso that c=1 when X is C and c=0 when X is N;
when c=1, Y and Z are electron-withdrawing groups selected from the group consisting of CN, SO2Rf, SO2R3, P(O)(OR3)2, CO2R3, P(O)R32, C(O)RfC(O)R3, and cycloalkenyl groups formed therewith wherein Rf is a perfluoroalkyl group of 1-10 carbons optionally substituted with one or more ether oxygens;
R3 is methyl or ethyl;
Y and Z are the same or different;
or, when c=0, Y may be an electron-withdrawing group represented by the formula xe2x80x94SO2Rfxe2x80x2 where Rfxe2x80x2 is the radical represented by the formula xe2x80x94(Rfxe2x80x3SO2Nxe2x88x92(M+)SO2)mRfxe2x80x2xe2x80x3 where m=0 or 1, and Rfxe2x80x3 is xe2x80x94CnF2nnxe2x80x94 and Rfxe2x80x2xe2x80x3 is CnF2n+1 where n=1-10, optionally substituted by one or more ether oxygens to form a reaction mixture;
introducing a free radical initiator;
reacting said reaction mixture to form an ionomer having a melting point of 150xc2x0 C. or greater.
The present invention further provides for an ionically conductive composition comprising the polymer of the invention and a liquid imibibed therewithin.
The present invention further provides for an electrode comprising at least one electrode active material, the ionomeric polymer of the present invention mixed therewith, and a liquid imbibed therewithin.
The present invention further comprises an electrochemical cell comprising a positive electrode, a negative electrode, a separator disposed between the positive and negative electrodes, and a means for connecting the cell to an outside load or source wherein at least one of the group consisting of the separator, the cathode, and the anode, comprises the ionically conductive composition of the invention.
For the purposes of the present invention, the term sulfonyl methide refers to a functional group wherein an ionically bonded carbon atom is also bonded to at least one fluoroalkylsulfonyl group, while the term sulfonyl imide refers to a functional group wherein an ionically bonded nitrogen atom is also bonded to at least one fluoroalkylsulfonyl group.
Surprisingly, the conductive compositions of the present invention are readily melt processible into electrodes and separators useful in assembling batteries in low cost continuous or semi-continuous manufacturing processes. No previous ionomer based composition suitable for use in electrochemical cells is known to exhibit melt processibility.
The ionomers of the present invention comprise monomer units derived from VF2 and 0.5-50 mol-%, preferably 2-20 mol-%, most preferably 3-12 mol-%, of ionic monomer units having pendant groups comprising the radical represented by the formula
xe2x80x94(OCF2CFR)aOCF2(CFRxe2x80x2)bSO2Xxe2x88x92(M+)(Y)(Z)c
wherein
R and Rxe2x80x2 are independently selected from F, Cl or a perfluoroalkyl group having 1 to 10 carbon atoms optionally substitued by one or more ether oxygens;
a=0, 1 or 2;
b=0 to 6;
M+ is H+ or a univalent metal cation;
X is C or N with the proviso that c=1 when X is C and c=0 when X is N;
when c=1, Y and Z are electron-withdrawing groups selected from the group consisting of CN, SO2Rf, SO2R3, P(O)(OR3)2, CO2R3, P(O)R32, C(O)RfC(O)R3, and cycloalkenyl groups formed therewith wherein Rf is a perfluoroalkyl group of 1-10 carbons optionally substituted with one or more ether oxygens;
R3 is an alkyl group of 1-6 carbons optionally substituted with one or more ether oxygens, or an aryl group optionally further substituted;
Y and Z are the same or different;
or, when c=0, Y may be an electron-withdrawing group represented by the formula xe2x80x94SO2Rfxe2x80x2 where Rfxe2x80x2 is the radical represented by the formula xe2x80x94(Rfxe2x80x3SO2Nxe2x88x92(M+)SO2)m Rfxe2x80x2xe2x80x3 where m=0 or 1, and Rfxe2x80x3 is xe2x80x94CnF2nxe2x80x94 and Rfxe2x80x2xe2x80x3 is xe2x80x94CnF2n+1 where n=1-10, optionally substituted by one or more ether oxygens.
Preferably, a=0 or 1, Rxe2x95x90CF3, Rxe2x80x2xe2x95x90F, b=1, and when X is C, Y and Z are CN or CO2R3 where R3 is C2H5, while when X is N, Y is preferably SO2Rf where Rf is CF3 or C2F5 and M+ is H+ or alkali metal cation. Most preferably M+ is a lithium cation. Most preferably the ionomer of the invention exhibits a melting point of 150xc2x0 C. or higher as determined by the peak of the endotherm as measured by differential scanning calorimetry (ASTMD4591).
The methide ionomers of the present invention may be formed by copolymerization of (II) with VF2 according to the teachings of Connolly, op.cit. Preferably, however, the methide ionomer is made by the process of the invention, wherein in a preparatory step is formed a copolymer of VF2 with the sulfonyl fluoride monomer (III).
The polymerization of (III) with VF2 may be conducted according to the teachings of Connolly et al, op. cit. Preferably, the polymerization is conducted with pre-emulsified liquid comonomer in a reaction mixture as taught hereinbelow. The ionomers formed from non-ionic polymer which has been polymerized in such fashion exhibit surprisingly high melting points of ca. 150xc2x0 C. or higher as determined from the peak of the endotherm in differential scanning calorimetry (ASTM D4591) in view of their bulk comonomer contents.
In the process of making the methide ionomer, the non-ionic sulfonyl fluoride copolymer, however formed, is then contacted in an inert organic liquid at a temperature of 0-150xc2x0 C., preferably 20-70xc2x0 C., with a carbanion derived from CH2YZ, wherein Y and Z are electron-withdrawing groups selected from the group consisting of CN, SO2Rf, SO2R3, P(O)(OR3)2, CO2R3, P(O)R32, C(O)Rf C(O)R3, and cycloalkenyl groups formed therewith wherein Rf is a perfluoroalkyl group of 1-10 carbons optionally substituted with one or more ether oxygens;
R3 is an alkyl group of 1-6 carbons optionally substituted with one or more ether oxygens, or an aryl group optionally further substituted;
Y and Z are the same or different.
Preferably Y and Z are CN or CO2R3 where R3 is C2H5, and the base used to generate reactive species from CH2YZ is preferably an alkali metal hydride, most preferably lithium hydride.
The combination is allowed to react until the sulfonyl fluoride is completely converted, which takes typically 15-20 hours in the preferred temperature range of 20-70xc2x0 C.
Most preferably, CH2YZ as hereinabove described, is combined with the copolymer of VF2 and (III), and lithium hydride in the inert organic liquid in the ratio of one gram equivalent weight of CH2YZ and two gram equivalent weights of lithium hydride per gram equivalent weight of sulfonyl fluoride.
Suitable inert organic liquids include oxygen-containing solvents such as dialkyl ethers, dimethoxyethane, tetrahydrofuran, dioxane, sulfolane, dimethyl sulfoxide, n-methyl pyrrolidone, dimethyl formamide, and acetonitrile. The preferred solvent will also be readily removed upon completion of the reaction. Preferred is dimethoxyethane.
The metal fluoride coproduct formed in the methidization process of the invention may be removed, if desired, by extraction or a dialysis process using water.
It is found in the practice of the invention, that in the formation of ionomers, the liquid medium in which the ionic species is formed often forms highly stable solvates therewith, making it difficult to fully remove that liquid by ordinary means such as drying or distillation. The residual liquid is preferably removed by addition of another metal ion ligating agent such as an organic carbonate, sulfolane, alkylphosphate, or dimethoxyethane which replaces the residual liquid, typically at moderately elevated temperatures in an anhydrous fluid such as toluene.
A monomeric form of the methide moiety of the ionomer of the invention may be formed by starting with the unsaturated olefinic structure (III), followed by bromination as is known in the art in order to protect the double bond, reaction as hereinabove described for the analogous copolymer, followed by treatment with Zn powder to yield the polymerizable double bond.
To form the imide ionomer of the present invention, VF2 is copolymerized with the monomeric composition represented by
CF2xe2x95x90CF(OCF2CFR)aOCF2(CFRxe2x80x2)bSO2N(M+)SO2Rfxe2x80x2xe2x80x83xe2x80x83(V)
R and Rxe2x80x2 are independently selected from F, Cl or a perfluoroalkyl group having 1 to 10 carbon atoms optionally substituted by one or more ether oxygens;
a=0, 1 or 2;
b=0 to 6;
M+ is H+ or a univalent metal cation, Rfxe2x80x2 is the radical represented by the formula xe2x80x94(Rfxe2x80x3SO2Nxe2x80x94(M+)SO2)mRfxe2x80x2xe2x80x3 where m=0 or 1, and Rfxe2x80x3 is xe2x80x94CnF2nxe2x80x94 and Rfxe2x80x2xe2x80x3 is xe2x80x94CnF2n+1 where n=1-10, optionally substituted by one or more ether oxygens.
Preferably, a=0 or 1, Rxe2x95x90CF3, Rxe2x80x2xe2x95x90F, b=1, and when X is Rf is CF3 or C2F5, and M+ is an alkali metal cation, most preferably lithium cation.
The olefinic monomer (V) may be synthesized according to the teachings of Xue, op.cit. The polymerization may be effected according to the teachings of Connolly et al, op. cit.
It is found in the practice of the present invention that the method by which the ionomer is formed can have a large effect on the melting temperature of the ionomer formed thereby. Melting point is of importance because a higher melting ionomer will provide a higher use temperature in such applications as lithium batteries.
The prior art teaches an aqueous emulsion process for copolymerizing methide or imide monomers according to DesMarteau or Xue, op. cit, with tetrafluoroethylene (TFE). Reaction kinetics dictate that the process of DesMarteau necessarily will result in limited, nearly random incorporation of the imide or methide monomers. An alternative though less convenient process known in the art, is to polymerize in a perfluorinated solvent.
Because of very substantial differences in reaction kinetics, the rate of incorporation and distribution of a comonomer in copolymerization with VF2 depends upon the availability of the comonomer in the aqueous polymerization medium. It has been found very surprisingly than when the methide and imide ionomers herein are copolymerized with VF2 in the aqueous emulsion polymerization of the art such as in Connolly et al, op. cit., ionically rich and ionically poor regions are developed. This results in an ionomer exhibiting a melting temperature higher than that achieved when an ionomer of the same over-all composition is formed by first copolymerizing VF2 with (III) using the same process followed by forming the ionomer.
An alternative means for providing the desired higher melting ionomer while avoiding the pitfalls of unwanted side reactions associated with polymerizing the ionic species, is to copolymerize VF2 with (III) in an aqueous medium wherein the liquid-liquid interface is substantially increased over that in the method of Connolly such as that in which the water, surfactant and monomer are pre-emulsified under very high shear mixing conditions as hereinbelow described.
Alternative means for achieving the high melting ionomers of the invention are available by copolymerizing VF2 with (III) in perfluorinated solvents, but this is less preferred because of the expense and handling difficulties inherent therewith.
It is found in the practice of the invention that it is preferred to make the methide ionomer by polymerizing VF2 with (III) in a pre-emulsified state as hereinbelow described, followed by forming the methide as hereinabove described. However, the imide is preferably formed by first forming the imide monomer according to Xue, op. cit., followed by polymerizing in an aqueous medium along the lines of Connolly, op.cit. In both approaches, the preferred method results in the preferred ionomer having a melting point of 150xc2x0 C. and above.
The imide analog of (II) may be synthesized by exposing a composition represented by the formula
CF2xe2x95x90CF(OCF2CFR)aOCF2(CFRxe2x80x2)bSO2Fxe2x80x83xe2x80x83(III)
made according to the teachings of Asahi Chemical Industry, GB 2051 831, 1980 (K. Kimoto, H. Miyauchi, J. Ohmura, M. Ebisawa et al.) to bromine or chlorine in an anhydrous inert atmosphere at a temperature of ca. 0xc2x0 C. in order to protect the olefinic bond according to the teaching in U.S. Pat. No. 5,463,005 forming thereby a composition represented by (IV). After washing to remove excess halogen using for example NaHSO3, the thus brominated starting material is combined under dry conditions, preferably in an anhydrous, aprotic organic solvent, with anhydrous MF, where M is K or Cs, and a composition represented by the formula Rfxe2x80x2SO2NH2, where Rfxe2x80x2 is the radical represented by the formula xe2x80x94(Rfxe2x80x2SO2Nxe2x80x94(M+)SO2)mRfxe2x80x2xe2x80x3 where m=0 or 1, or possibly  greater than 1, and Rfxe2x80x3 is CnF2n and Rfxe2x80x2xe2x80x3 is represented by the formula CnF2n+1 where n=1 to 10, RfSO2NH2 being made according to the teachings of Meuxcex2endoerffer and Niederprxc3xcm (Chemiker Zeitung, 96. Jahrgang (1972) No. 10, 583). Suitable solvents include acetonitrile, dioxane, and sulfolane.
Preferably Rfxe2x80x2 is CnF2n+1 where n=1 to 4.
The mixture thus formed is heated to a temperature in the range of 50-150xc2x0 C., preferably 70-90xc2x0 C., and the reaction is allowed to proceed preferably until the Rfxe2x80x2SO2NH2 has been consumed as determined by NMR. Upon termination of the reaction, the product, which remains in solution is separated by filtration. In order to regenerate the olefinic bond, the reaction product is then contacted with metallic zinc, preferably by slurrying Zn powder into the solution, at ambient temperature and then heated for several hours, as taught in U.S. Pat. No. 5,463,005, preferably followed by filtering and washing with an anhydrous, aprotic organic solvent such as acetonitrile. Thus formed is a composition represented by the formula:
CF2xe2x95x90CF(OCF2CFR)aOCF2(CFRxe2x80x2)bSO2N(M)SO2Rfxe2x80x2xe2x80x83xe2x80x83(V)
The lithiated imide form of structure (V) is then copolymerized with VF2 according to the teachings of Connolly et al. Unlike the methide embodiment, wherein it is preferred to first make a copolymer of VF2 and (II) followed by methidization, in the case of the imide it is highly preferred to first make the imidized monomer (V) followed by polymerization with VF2.
In many applications, the ionomer is preferably formed into a film or sheet. Films may be formed according to processes known in the art. In one embodiment, the ionomer is diluted with a solvent such as DMAC, the mixture cast onto a smooth surface such as a glass plate using a doctor knife or other device known in the art to assist in depositing films on a substrate, and the solvent evaporated. Preferably the ionomer of the invention is first combined with a plasticizer and then is formed into a film or sheet by a melt process. Most preferably, the melt process is an extrusion process.
The ionomers of the present invention, however formed, may exhibit a low level of ionic conductivity in the dry state, typically about 10xe2x88x927 S/cm at room temperature. The ionomer may be combined with a liquid to achieve higher levels of ionic conductivity. Depending upon the requirements of the application, the ionomer will be in the acid form or the metal salt form, the particular metal being determined by the application as well. The liquid employed therewith will likewise be dictated by the application. In general terms, it has been found in the practice of the invention that conductivity of the liquid-containing ionomer increases with increasing percent weight uptake, increasing dielectric constant, and increasing Lewis basicity of the liquid, while conductivity has been observed to decrease with increasing viscosity and increasing molecular size of the liquid employed. Thus, a highly basic solvent of low viscosity and small molecular size but low dielectric constant may provide superior conductivity in a given membrane than a larger, more viscous, less basic solvent of very high dielectric constant. Of course, other considerations may come into play as well. For example, the liquid may be electrochemically unstable in the intended use.
Conductive compositions may thus be formed by combining together the ionomers of the present invention with solvents using a variety of techniques known in the art such as imbibing a dry ionomer film in a mixture of solvents or exposure of a dry film to a solvent vapor under controlled conditions or combining the ionomer with the solvents in a melt state and extruding films of controlled composition. Preferred solvents include water, nonaqueous solvents such as linear and cyclic carbonates, alcohols, esters, lactones, ethers, sulfoxides, amides, sulfonamides, and sulfones, subject to the general considerations discussed above. The solvents combined with the ionomers of the present invention to form conductive compositions can optionally contain additional mobile salts which may be preferred for specific applications. Other solvents suitable for forming conductive compositions include ionic liquids such as 1-methyl-3-butyl-imidazolium trifluoromethane sulfonate.
A variety of chemical agents can be added to these conductive compositions for purposes of improving ionic conductivity through the influence of the chemical agent on the dissociation or mobility of the ions within the ionomeric polymer. Such chemical agents include but are not limited to cationic complexing agents such as crown ethers and aza ethers and anion complexing agents such as BR3 compounds where R is aryl, fluoro-substituted alkyl or aryl.
The ionomers of the present invention provide several unexpected benefits over the ionomers of the art. It is known in the art that VF2 polymers and copolymers exhibit electrochemical stability which makes them structural materials of choice for use in lithium batteries. Compared to the ionomers in the art which contain fluorosulfonate salts, the ionomers of the present invention comprise fluorosulfonylmethide or imide salts which exhibit higher dissociation in organic solvents thereby providing conductive compositions formed therefrom with surprisingly high conductivity. The preferred conductive compositions of the present invention, comprising the lithium salt embodiments of the ionomers of the invention and aprotic organic solvents, most preferably organic carbonates and lactones, are particularly well-suited for use in lithium batteries.
In an additive effect thereto, it is found, surprisingly, that the ionomers of the present invention exhibit particularly high affinity and phase compatibility with organic solvents as compared to the ionomers of DesMarteau, op. cit., formed with TFE. The higher affinity of the ionomers of the invention to organic solvents on the one hand makes melt processing or casting of membranes a useful process for the production thereof; and, on the other hand, provides for higher uptake of the preferred organic carbonates in the preferred conductive compositions of the invention, leading to higher conductivities thereby.
It is found in the practice of the invention that certain compositions of an ionomer of the invention containing at least 50% VF2 more preferably at least 80% VF2 may become plasticized by the solvents imbibed within it, with concomitant decrease in mechanical strength of the membrane. In some applications, it may be desirable to enhance the properties of the solvent-swollen membrane. Means available for improving the mechanical properties include: 1) incorporation into the polymer by means known in the art, a non-ionic third monomer that is not solvent sensitive; 2) formation by known means of a polymer blend with a non-ionic polymer that is less solvent sensitive; 3) blending by known means of the ionomer of the invention with an inert filler; 4) blending different compositions of ionic copolymers.
In a preferred embodiment of this invention involves the use of compositionally heterogeneous xe2x80x94SO2Fxe2x80x94 containing copolymer as precursor for the ionomeric form. Combined attributes of increased conductivity and enhanced mechanical strength are thereby obtained.
Suitable third monomers which may be usefully incorporated in these ionomeric compositions include tetrafluoroethylene, chlorotrifluoroethylene, ethylene, hexafluoropropylene, trifluoroethylene, vinyl fluoride, vinyl chloride, vinylidene chloride, perfluoroalkylvinyl ethers of the formula CF2xe2x95x90CFORf where Rfxe2x95x90CF3, C2F5 or C3F7. Preferred termonomers include tetrafluoroethylene, hexafluoropropylene, ethylene and the perfluoroalkylvinyl ethers. Termonomers are preferably present in the polymer at a concentration of up to 30 mol-%.
Polymers suitable for blending with ionomers of the invention include poly(tetrafluoroethylene) and copolymers thereof with hexafluoropropylene or perfluoroalkyl vinyl ethers, polyvinylidene fluoride homopolymer and a copolymer thereof with hexafluoropropylene, and polyethylene oxide. A preferred composition comprises 25 to 50 weight % PVF2 homopolymer blended with the VF2 ionomer of the present invention. These materials are blended together by means common in the art such as mixing in a common diluent such as DMAC or propylene carbonate and then casting a membrane.
Suitable inert fillers include SiO2, Al2O3, TiO2, or CaF2. High surface area particles less than 1.0 micron in diameter are desired, such as are available for the preferred grade of SiO2 under the trade name Cab-o-sil(copyright) TS-530 silica. Loadings of up to 50 weight % filler are preferred.
The preferred electrode of the invention comprises a mixture of one or more electrode active materials in particulate form, the ionomer of the invention, at least one electron conductive additive, and at least one organic carbonate. Examples of useful anode active materials include, but are not limited to, carbon (graphitic, coke-type, mesocarbon microbeads, carbon fibers, polyacenes, and the like) and lithium-intercalated carbon, lithium metal nitrides such as Li2.6Co0.4N, lithium metal, and lithium alloys, such as alloys of lithium with aluminum, tin, magnesium, mercury, manganese, iron, antimony, cadmium, and zinc, alloy forming anode compounds with inert metallic frameworks such as tin-iron-carbon or tin-manganese-carbon ternary compounds, metal oxides or lithium metal oxides such as tin oxide, iron oxide, titanium oxide, tantalum oxide, niobium oxide, or tungsten oxide, and electronically anion or cation-doping conductive polymers such as polyaniline. Lithium intercalation anodes employing graphitic carbon such as MCMB 2528 from Osaka Gas Chemical Co. are preferred.
Useful cathode active materials include, but are not limited to, transition metal oxides such as spinel LiMn2O4, layered LiMnO2, LiNiO2, LiCoO2, LiNixCoyO2, iron oxides or lithiated iron oxides such as LiFeO2, or vanadium oxides such as LiV2O5, LiV6O13, LiNiVO4, LiCoVO4, or the above compounds in nonstoichiometric, disordered, amorphous, or overlithiated or underlithiated forms (such as having metallic vacancies, oxygen vacancies or defects, etc.), the above compounds doped with small amounts of other divalent or trivalent metallic cations such as Fe2+, Ti2+, Zn2+, Ni2+, Co2+, Cu2+, Cr3+, Fe3+, Al3+, Ni3+, Co3+, Mn3+, etc., sulfur compounds such as solid sulfur, organic disulfides, or metal sulfides such as TiS2 or MoS2, electronically-conducting polymers such as polyaniline and its derivatives, polypyrrole derivatives, polyparaphenylene derivatives, polythiophene derivatives, or their copolymers, or mixtures of any of the above compounds. Particle size of the active material should range from about 1 to 100 microns. Preferred are transition metal oxides such as LiMn2O4, LiNiO2, LiCoO2, and LiNixCoyO2. A highly preferred electron conductive aid is carbon black, preferably Super P carbon black, available from the MMM S.A. Carbon, Brussels, Belgium, in the concentration range of 1-10%. Preferably, the volume fraction of the lithium ionomer in the finished electrode is between 4 and 40%.
The electrode of the invention may conveniently be made by dispersion or dissolution of all polymeric components into a common solvent and mixing together with the electrode active particles the carbon black particles. For cathodes the preferred electrode active material is LiNixCo1xe2x88x92xO2 wherein 0 less than x less than 1, while for anodes the preferred electrode active material is graphitized mesocarbon microbeads. For example, a preferred lithium battery electrode of the invention can be fabricated by dispersing or dissolving ionomer of the invention in a mixture of propylene carbonate and cyclopentanone, followed by addition of particles of electrode active material and carbon black, followed by deposition of a film on a substrate and drying. Preferably, the components of the electrode are mixed together and fed to an extruder wherein they are mixed to form a homogeneous melt and extruded into a film.
The resultant preferred electrode will comprise electrode active material, conductive carbon black, and ionomer of the invention, where, preferably, the weight ratio of ionomer to electrode active material is between 0.05 and 0.8 and the weight ratio of carbon black to electrode active material is between 0.01 and 0.2. Most preferably the weight ratio of ionomer to electrode active material is between 0.1 and 0.25 and the weight ratio of carbon black to electrode active material is between 0.02 and 0.1. This electrode can then be cast from solution onto a suitable support such as a glass plate, inert polymer carrier web, or current collector metal foil, and formed into a film using techniques well-known in the art. The electrode film thus produced can then be incorporated into a multi-layer electrochemical cell structure by lamination.
Battery solvents may be added to the battery component films individually or added to the battery laminated cells using a variety of techniques known in the art such as imbibing by immersion into a solution or exposure to solvent vapors under controlled conditions. Preferred battery solvents for forming conductive compositions with the ionomeric polymers of the present invention suitable for usage in lithium batteries include dipolar aprotic liquids such as the linear and cyclic carbonates, esters, lactones, amides, sulfoxides, sulfones, sulfamides, and ethers. Preferred solvents are mixtures of cyclic carbonates or lactones such as ethylene carbonate, propylene carbonate, butylene carbonates, vinylene carbonate, gamma-butyrolactone, fluoro or chloro-substituted cyclic carbonates with linear carbonates such as dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, and fluoro and chloro substituted linear carbonates. Especially preferred are mixtures of ethylene carbonate, propylene carbonate, and gamma-butyrolactone with linear carbonates such as diethyl carbonate and/or ethyl methyl carbonate. Most preferred are mixtures of ethylene carbonate with propylene carbonate in weight ratios of from 50:50 to 80:20 of ethylene carbonate to propylene carbonate.
These solvents can optionally be combined with additional mobile salts such as the lithium salts LiPF6, LiPFxRfy where Rf=CF3, CF2CF3, or other perfluorinated electron-withdrawing groups, LiBF4, LiAsF6, LiClO4, LiSO3Rf where Rf=CF3, CF2CF3, or other perfluorinated electron-withdrawing groups, LiN(SO2R1)(SO2R2) where R1 and R2=CF3, CF2CF3, or other electron-withdrawing groups and R1 is not necessarily the same as R2, LiC(SO2R3)(SO2R4)(SO2R5) where R3, R4, and R5=CF3, CF2CF3, or other electron-withdrawing groups and R3, R4 and R5 are not necessarily the same and mixtures of the above salts. Preferred are LiPF6 or LiN(SO2CF2CF3)2.
In a preferred embodiment of the battery of the present invention, a battery is formed from one or more electrochemical cells formed by laminating together in film form the anode, cathode, and separator compositions of the present invention, all of which have been rigorously dried prior to addition of a liquid selected from the group of organic carbonates and mixtures thereof, a mixture of ethylene carbonate and propylene carbonate being most preferred.
In a more preferred embodiment of the battery of the present invention, the individual film layers consisting of an anode, separator, and cathode are compounded individually in a melt state and extruded into film form using temperatures from 90 to 130xc2x0 C. These individual layers already containing the preferred battery solvents such as mixtures of ethylene carbonate and propylene carbonate are laminated together to form battery cells which do not require additional post-treatment such as drying or extraction steps.
It may be desirable to incorporate into the electrode composition of the invention additional polymers or solvents for such purposes as improving the binding of the components thereof, or providing improved structural integrity of an article fabricated therefrom. One particularly preferred additional material is PVF2 homopolymer, which may be incorporated simply by dissolving the polymer into the same solution from which the electrode is being formed or melt compounding the polymer into other components during mixing or extrusion, as hereinabove described.